1. Field of the Invention
The present invention relates generally to a machine having a mold consisting of a stationary die and a movable die which are clamped together to produce a formed product, and more particularly to an improved mold clamping device for clamping the mold.
2. Discussion of the Prior Art
As a machine for producing a formed product by molding, casting or pressing using a mold consisting of a stationary die and a movable die, there is widely known a forming machine of a mold-clamping type such as a casting machine for aluminum alloys, in which the stationary and movable dies are clamped together by a clamping force. This mold-clamping type of forming machine includes (a) a stationary die plate to which the stationary die is fixed, (b) a movable die plate which carries the movable die such that the movable die faces the stationary die, and (c) a clamping hydraulic cylinder for moving the movable die plate toward and away from the stationary die plate. In this forming machine, an operation to produce a formed product such as a casting, molding or pressing is effected while the mold is in the clamped state, namely while the stationary and movable dies are clamped together. An example of the known mold-clamping type forming machine is disclosed in JP-A-3-155445 (laid-open publication of unexamined Japanese Patent Application), and is illustrated in FIG. 9, wherein a plurality of tie rods or guide posts 3 are fixed to a stationary die plate 2 on which a stationary die 1 is fixedly disposed. The guide posts 3 extend upright from the stationary die plate 2, so that a movable die plate 5 which carries a movable die 4 is slidably guided by the guide posts 3 in the vertical direction. The movable die plate 5 is moved in the vertical direction by a mold closing and opening hydraulic cylinder 7 mounted on a bracket 6 which is attached to the upper ends of the guide posts 3. After the movable die 4 is brought into abutting contact with the stationary die 1 by the hydraulic cylinder 7, the movable die plate 5 is pressed toward the stationary die plate 2, also by the hydraulic cylinder 7, to force the movable die 4 against the stationary die 1, that is, to clamp the mold 1,4.
In some operations of such mold-clamping type forming machines as described above, a relatively large clamping force must be applied to the mold. For instance, where a mold cavity defined by the stationary and movable dies in abutting contact with each other is filled with a molten metal for casting, pressurizing cylinders incorporated in the mold are activated to apply a high pressure to the mass of the molten metal in the mold cavity. In this case, the mold closing and opening hydraulic cylinder should have a sufficiently large pressure receiving area. This means that a comparatively large amount of a working fluid should be supplied to the hydraulic cylinder to move the movable die plate for closing and opening the mold, whereby the operating cycle time of the machines tends to be long, with a result of lowering the production efficiency. Although the cycle time can be shortened by using a hydraulic pump with a large delivery capacity for the mold closing and opening hydraulic cylinder, this leads to increased size and cost of the hydraulic system.
Another form of the mold-clamping type forming machine as indicated above is disclosed in laid-open Publication No. 64-10394 of unexamined Japanese Utility Model Application. This forming machine is a pressing machine which includes (a) a stationary die plate to which a stationary die is fixed, (b) a plurality of guide posts attached upright to outer portions of the stationary die outside the periphery of the stationary die, (c) a plurality of mold clamping hydraulic cylinders having respective pistons provided as integral parts of the respective guide posts, and respective cylinder housings which axially slidably engage the respective guide posts and pistons, and (d) a movable die plate which is fixed to the cylinder housings of the mold clamping hydraulic cylinders and which carries a movable die such that the movable die faces the stationary die. The movable die plate is forced toward the stationary die plate by the mold clamping hydraulic cylinders, so as to clamp the stationary and movable dies together when a formed product is produced. This pressing machine in which the guide posts are utilized as part of the mold clamping hydraulic cylinders is simpler and more compact in construction than a machine in which the mold clamping hydraulic cylinders is/are mounted on a bracket fixed to the upper ends of the guide posts.
In the pressing machine of the above type in which the cylinder housings of the mold clamping hydraulic cylinders are fixed to the movable die plate, the movable die plate tends to undergo buckling or bending deformation when the outer portions of the movable die plate outside the periphery of the movable die are forced toward the stationary die plate to clamp the mold. In this event, the attitude of the cylinder housings fixed to the buckled movable die plate is more or less changed, whereby the cylinder housings tend to exert a stress on the guide posts and the pistons, resulting in damaging the cylinder housings, guide posts and pistons, and an early local wear of the sliding components, leading to shorted life expectancy of the machine.
Where the forming machine of the mold-clamping type as shown in FIG. 9 is used for casting or molding, the single mold closing and opening hydraulic cylinder 7 forces a central portion of the movable die plate 5. This arrangement does not assure even distribution of the clamping force over the entire contacting surfaces of the movable and stationary dies 1, 4, which are outside the periphery of the mold cavity which is to be filled with a molten material such as a molten metal. Consequently, the clamped mold may have a local gap between the contacting surfaces of the two dies. In this event, the molten material tends to penetrate or leak through the gap, and the formed product may have burrs due to the penetration or leakage of the molten material. This problem is serious particularly on a casting machine in which a molten metal in the mold cavity is pressurized by suitable means.
While the forming machine disclosed in the above-identified publication No. 64-10394 uses hydraulic cylinders to force outer portions of the movable die plate toward the stationary die plate, this forming machine is a pressing machine adapted to form plates or other planar workpieces under pressure, and does not suffer from the above problem associated with the molten material in the mold cavity.